In recent years, steel for line pipe has been required to be improved in strength so as to improve safety, raise the pressure of transported gas and thereby improve operating efficiency, and reduce the steel materials used so as to lower costs. Further, the regions in which such steel materials are being used are spreading to artic regions and other regions where the natural environment is harsh. Strict toughness characteristics are being required. Further, in steel for structures used in earthquake prone areas etc., in addition to the conventionally required characteristics, plastic deformation ability, ductile fracture resistance characteristics, etc. are sought.
For example, PLT 1 proposes steel suppressing ductile fracture by raising the uniform elongation. It uses the quenching, lamellarizing, and tempering process (QLT process) to mix a suitable amount of hardened phases in the ferrite to obtain a mixed structure and realize a high ductility. Further, PLT 2 realizes high ductility by optimization of the steel composition and quench hardenability (Di) and by accelerated cooling.
In general, in high strength steel, raising the carbon equivalent and hardenability index is considered necessary. However, when simply raising the carbon equivalent, a drop in the ductility and toughness is invited. On the other hand, with steel plate for large-size line pipe, it is required to reduce the variations in strength, ductility, etc. in the plate so as to manage the ductility after pipemaking such as UOE, JCOE, etc.